simulate in stats

BTW, I think there is a problem with the way the argument "seed" is  
used
in the new simulate in stats.  The problem is that repeated calls to
simulate using the default argument will introduce a new pattern into
the RNG:

stats:::simulate

function (object, nsim = 1, seed = as.integer(runif(1, 0,
.Machine$integer.max)),   ...)
UseMethod("simulate")
<environment: namespace:stats>

stats:::simulate.lm

function (object, nsim = 1, seed = as.integer(runif(1, 0,
.Machine$integer.max)),    ...)
{
    if (!exists(".Random.seed", envir = .GlobalEnv))
        runif(1)
    RNGstate <- .Random.seed
    set.seed(seed)
  ...

This should not be done, as the resulting RNG has not been studied or
proven. A better mechanism is  to have a default argument equal NULL,
and not touch the seed in that case. There are several examples of  
this
in the package dse1 (in bundle dse),  see for example simulate.ARMA  
and
simulate.SS. They also use the utilities in the setRNG package to save
more of the information necessary to reproduce simulations. Roughly it
is done like this
simulate.x <- function (model, rng = NULL,  ...)
  {if (is.null(rng)) rng <- setRNG() #returns the RNG setting to be
saved with the result
    else {
        old.rng <- setRNG(rng)
        on.exit(setRNG(old.rng))
        }
   ...


The seed by itself is not very useful if the purpose is to be able to
reproduce things, and I think it would be a good idea to  
incorporate the
few small functions setRNG into stats (especially if the simulate
mechanism is being introduced).

The argument "nsim" presumably alleviates to some extent the above
concern about changing the RNG pattern. However, in my fairly  
extensive
experience it is not very workable to produce all the simulations and
then do the analysis of them. In a Monte Carlo experiment the  
generated
data set is just too big. A better approach is to do the analysis and
save only necessary information after each simulation. That is the
approach, for example, in dse2:::EstEval.

Paul

Paul Gilbert wrote:

Can the arguments nsim and seed be passed as part of ... in the new
simulate generic in R-2.2.0alpha package stats?

This would potentially allow me to use the stats generic rather than
the one I define in dse. There are contexts where nsim and seed do  
not
make sense. I realize that the default arguments could be ignored,  
but
it does not really make sense to introduce a new generic with that in
mind. (I would also prefer that the "object" argument was called
"model" but this is less important.)

Paul Gilbert